A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Known lithographic apparatuses have an illumination system which provides imaging offering σ<1, wherein σ is a ratio between the numerical aperture of the illumination system that illuminates the patterning device pattern with the beam of radiation, and the numerical aperture of a projection system that projects the image of the patterning device pattern onto the resist layer.
The illumination system has a pupil, which is a Fourier transform of the object plane in which the patterning device of the lithographic apparatus is located. The pupil plane of the illumination system is conjugate to a pupil plane of the projection system. An illumination mode can be described by reference to the spatial distribution of intensity of a radiation beam in the pupil plane of the illumination system. It will be understood that the spatial distribution of intensity in the pupil plane of the projection system will be generally the same as the distribution of intensity in the pupil plane of the illumination system, subject to diffraction effects which may be caused by a pattern of the patterning device.